1. Field of the Invention
The present invention relates to a silicon resonant type pressure sensor in which the output signal can be doubled, and the S/N ratio is enhanced by increasing the signal level.
2. Description of the Related Art
JP-UM-A-4-116748 (pages 4 and 5, FIGS. 4, 6, and 7) is referred to as a related art of a silicon resonant type pressure sensor.
FIG. 8 is a diagram illustrating the configuration of main portions of an example of a related pressure sensor which is usually used. Such a pressure sensor is disclosed in, for example, JP-UM-A-4-116748 entitled “Silicon resonant type pressure measuring apparatus”.
FIG. 9 is an enlarged detail view of main portions of FIG. 8, FIG. 10 is an enlarged detail view of the silicon sensor chip of FIG. 9, FIG. 11 is a diagram illustrating the cross-section of a silicon resonator and the operation of FIG. 8, FIG. 12 is a diagram illustrating the operating principle of FIG. 8, and FIG. 13 is a schematic circuit diagram of a silicon resonator of FIG. 12.
In this example, a silicon resonant type pressure sensor is used in a pressure measuring apparatus.
In the figures, the reference numeral 1 is a sensor chip made of a single crystal silicon.
The reference numeral 2 is a sensing diaphragm which is formed by a dint 3 formed in the sensor chip 1, and which detects a measuring pressure Pm.
The reference numeral 4 is a strain detection sensor which is embedded in the sensing diaphragm 2. A silicon resonator is used as the strain detection sensor.
The reference numeral 5 is a shell which is fabricated by a semiconductor epitaxial growth layer for vacuum sealing, and which seals the silicon resonator 4 into the sensing diaphragm 2.
The reference numeral 6 is a vacuum chamber. The silicon resonator 4 is embedded into the vacuum chamber 6 formed in the sensing diaphragm 2.
As shown in FIGS. 11 and 12, the silicon resonator 4 is caused by a magnetic field generated from a permanent magnet 7, and a closed-loop self-excited oscillator circuit connected to the silicon resonator 4, to vibrate at the resonant frequency of the silicon resonator 4.
The permanent magnet 7 is mounted so as to be faced to the silicon resonator 4 via yokes 8, a yoke holder 9, and a spacer 11.
In FIG. 12, a silicon resonator has an H-like shape.
The reference numeral 21 is a metal cap having a U-like cross-section shape in which the yoke holder 9 is clearance-fitted to the side of a bottom portion 22, the bottom portion 22 pushes the yoke holder 9 against the spacer 11 in the direction to the silicon resonator 4, and a support member 12 is clearance-fitted to the side of an opening 23. An opening edge 24 of the metal cap is fixed by welding 25 to the support member 12.
The metal cap 21 is made of a material having a thermal expansion coefficient which is similar to the thermal expansion coefficients of the yoke holder 9 and the support member 12, and therefore accurately sets the relative positions of the silicon resonator 4 and the permanent magnet 7.
The reference numeral 26 is a hole which is formed in a middle area of the bottom portion 22.
In the above configuration, when the measuring pressure Pm is applied to the sensing diaphragm 2, the axial force of the silicon resonator 4 is changed, and the resonant frequency is changed. Therefore, the measuring pressure Pm can be measured in accordance with the change of the resonant frequency.
In this case, a driving current i0 is supplied to the excitation side of the silicon resonator 4 to vibrate the H-shaped resonator 4, and the resonant frequency is detected in accordance with an induced electromagnetic output voltage eout which is generated in the detection side.
As shown in FIG. 13, R1 is the resistance of the silicon resonator 4, and R2 is the resistance of the shell S.
In such an apparatus, the driving current i0 is supplied to the excitation side of the silicon resonator to cause the H-shaped resonator 4 to vibrate. The frequency is detected in accordance with the induced electromagnetic output voltage eout which is generated in the detection side. The input beam is separated from the output beam.
Because of restrictions on the shape of the resonator, the immensity of the magnetic flux density, and the like, the level of the output voltage eout is limited.
Even when the resonator is miniaturized so as to be smaller than that of the related art, the level of the output voltage becomes smaller, and the S/N ratio also become worse, so that the signal detection is hardly conducted. Therefore, it is difficult to miniaturize the resonator size.
In summary, because of restrictions on the shape of the resonator, the magnetic flux density, and the like, the level of the output voltage eout is small.
Since the resonator and the shell portion are not electrically isolated from each other, the driving current i0 flows to the shell.
Since the excitation and detection sides of the resonator are not electrically isolated from each other, the driving current i0 leaks to the detection side, consequently the cross talk level is increased.
As a result, the S/N ratio of the output voltage of the resonator 4 becomes worse.